1. Field of the Invention
This invention lies in the field of exhaled breath analyzers for monitoring pulmonary conditions.
2. Description of the Prior Art
Persons suffering from asthma are able to monitor the intensity of their condition and to predict the likelihood of an asthmatic attack by monitoring the level of nitric oxide (NO) in their exhaled breath. Instruments that detect nitric oxide levels in exhaled breath are disclosed in Silkoff, U.S. Pat. No. 5,795,787, issued Aug. 18, 1998; Silkoff et al., U.S. Pat. No. 6,010,459, issued Jan. 4, 2000; Parikh et al., United States Pre-Grant Publication No. US 2004-0017570 A1, published Jan. 29, 2004; Flaherty et al., United States Pre-Grant Publication No. US 2005-0083527 A1, published Apr. 21, 2005, and corresponding U.S. Pat. No. 7,220,387, issued May 22, 2007; Parikh et al., United States Pre-Grant Publication No. US 2005-0053549 A1, published Mar. 10, 2005; Flaherty et al., US 2007-0261472 A1, published Oct. 15, 2007; Nason et al., United States Pre-Grant Publication No. US 2006-0195040 A1, published Aug. 31, 2006; Chazan, United States Pre-Grant Publication No. US 2006-0177890 A1, published Aug. 10, 2006; and Anvar et al., United States Pre-Grant Publication No. US 2007-0086920 A1, published Apr. 19, 2007. The instruments described in these documents contain nitric oxide sensors retained in flow-through passages for direct contact with an individual's exhaled breath, together with pumps, filters, and various detectors and controls that allow the user to detect nitric oxide levels in the breath, and changes in such levels, in the parts-per-billion range.
The sensor is typically a nitric oxide-binding protein such as cytochrome C that undergoes an optically detectable change upon binding to nitric oxide. To achieve high sensitivity and stability, the protein is encapsulated in a support matrix at a controlled concentration, the matrix having a controlled porosity and surface area. Preferred matrices are xerogels and aerogels. Despite these controls, the sensor is subject to variability since the binding equilibria between the protein and the nitric oxide are significantly affected by other components in the system, even those in minute amounts, including extraneous species present in the breath samples, the atmosphere, and the instrument itself. As a result, the typical sensor is conditioned for use by the manufacturer and then used only once, requiring a fresh sensor for each use.